Shear-activated viscoelastic surfactant fluid and method

ABSTRACT

A well treatment composition is formed from a fluid mixture of a viscoelastic surfactant and a liquid carrier fluid. The fluid mixture has rheological properties wherein the mixture exhibits shear-thickening behavior when the shear rate is increased from a first shear rate to a second higher shear rate. The fluid mixture may further include a shear activation additive that interacts with the viscoelastic surfactant to facilitate the shear-thickening behavior. The method is accomplished by introducing the fluid mixture into a wellbore formed in a subterranean formation. In certain applications, the fluid mixture may be recycled by bringing the fluid mixture to the surface and reintroducing the fluid into the same or a different wellbore.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

In the construction and development of wells formed in subterraneanformations, such as wells for the production of oil and gas, variousoperations are carried out that require the introduction of fluids ofdifferent types into the wellbore and/or into formation surrounding thewellbore.

In certain applications it may be desired to pump or introduce a thinfluid and then rely on some trigger to make the fluid more viscous orviscoelastic to perform its intended function.

Viscoelastic surfactants (VES) have been used in fluids for many welltreatment operations. Not to be limited by theory, but theseviscoelastic surfactant systems under proper conditions form longrod-like or worm-like micelles in aqueous solution. Entanglement ofthese micelle structures gives viscosity and elasticity to the fluid.When these fluids are subjected to shear, the shear disrupts the micellenetwork so that the fluids exhibit shear-thinning behavior. When theshear is removed or lessened, the viscosity of the fluid recovers as theVES micelle network reassembles.

This shear-thinning behavior is beneficial in many well treatments. Inhydraulic fracturing, for example, shear-thinning behavior is desired asthe fracturing fluid is being pumped into the wellbore. Once thefracturing fluid enters the formation, however, increased viscosity isdesired to facilitate suspension of proppant into the formed fractures.Thus, in these VES fluids the reduced shear results in an increase inviscosity of the fluid.

In coiled tubing cleanouts, it is desirable to introduce a thin fluid tominimize pumping power requirements, but then provide a thicker fluidonce the fluid is jetted from the coiled tubing nozzle to suspend andtransport particles back to the surface to facilitate cleanout. In thiscase, the VES fluid is thinned during the high shear imparted as thefluid is being pumped down the wellbore tubulars. Once jetted, theviscosity of the fluid recovers to facilitate cleanout.

U.S. Pat. No. 7,290,615 describes an example of a VES fluid that may beused for cleanout applications and that may be recycled for repeateduse. In such applications, the pH of the fluid is cycled by utilizing pHadjusting agents to provide higher or thinner viscosity. Thus, when theVES fluid is introduced into the wellbore for cleanout, the fluid isprovided with a high pH that provides a high viscosity for suspendingparticles cleaned from the well. When the cleanout fluid with theentrained particles is brought to the surface, the pH of the fluid islowered so that the viscosity of the fluid is decreased to facilitatesettling of the entrained particles. After the particles have settled,the fluid viscosity can then be increased again by readjusting the pH.The fluids may thus be used repeatedly by cycling the pH to differentlevels.

As can be seen, while the shear-thinning behavior of these VES fluids isbeneficial, there may be applications where increases in viscosity ofthe VES fluid are necessary or beneficial in high shear environments orwhere an increase in shear may trigger an increase in viscosity withoutrequiring chemical modification of the fluid. The present embodimentsare directed to providing such fluids and methods.

SUMMARY

A well treatment method is carried out by providing a fluid mixture of aviscoelastic surfactant and a liquid carrier fluid. The fluid mixturehas rheological properties wherein the mixture exhibits shear-thickeningbehavior when the shear rate is increased from a first shear rate to asecond higher shear rate. In certain embodiments, the first shear rateis from about 10 s⁻¹ to about 100 s⁻¹. In the method, the fluid mixtureis introduced into a wellbore formed in a subterranean formation.

In certain embodiments, the viscoelastic surfactant is present in theliquid carrier fluid in a state wherein the viscoelastic surfactantexhibits general phase separation within the carrier fluid when atstatic surface conditions. The fluid mixture may be introduced into awellbore as at least one of wellbore cleanup fluid, a diverter fluid, afracturing fluid and a fluid loss pill.

The fluid mixture may further comprise a rheology enhancer. The rheologyenhancer may be selected from at least one of a polyvinyl ester, apolyvinyl acetate, a polyacrylate, a polyvinyl alcohol, an aromaticsulfonate, mutual solvents, ABA copolymers, and combinations thereof.

The viscoelastic surfactant may be selected from a zwitterionicsurfactant, an amphoteric surfactant, an anionic surfactant, a cationicsurfactant, and combinations thereof. The zwitterionic surfactant maycomprise a surfactant or mixture of surfactants having the formula:)

RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH²)_(b)—N⁺(CH₃)₂—(CH₂)_(a)′(CH₂CH₂O)_(m)′(CH₂)_(b)′COO⁻

wherein R is an alkyl group that contains from about 17 to about 23carbon atoms which may be branched or straight chained and which may besaturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and mand m′ are each from 0 to 13, a and b are each 1 or 2 if m is not 0 and(a+b) is from 2 to 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is 0and (a′+b′) is from 1 to 5 if m′ is 0; (m+m′) is from 0 to 14; andCH₂CH₂O may also be OCH₂CH₂.

The fluid mixture may further comprise a shear activation additive thatinteracts with the viscoelastic surfactant to facilitate theshear-thickening behavior. In certain applications, the shear activationadditive may be a delayed shear activation additive so that shearactivation additive interacts with the viscoelastic surfactantsubsequent to the fluid mixture being introduced into the wellbore.Where the viscoelastic surfactant is a zwitterionic surfactant, theshear activation additive may be a base.

In certain applications, the fluid mixture introduced into the wellboreis brought to the surface and reintroduced into the same or a differentwellbore. In some cases, the fluid mixture introduced into the wellboreis allowed to entrain particles from the well within the fluid mixture.The fluid mixture is brought to the surface at a flow rate thatfacilitates the shear thickening behavior when brought to the surfacewhile the particles are entrained therein. The fluid mixture is allowedto rest after being brought to the surface so that the viscosity of thefluid mixture falls below the viscosity exhibited during the shearthickening behavior so that the entrained particles settle within thefluid mixture. The fluid mixture may then be reintroduced into the sameor a different wellbore after separating the settled particles from thefluid mixture.

In still another embodiment, a well treatment method is carried out byproviding a fluid mixture of a zwitterionic viscoelastic surfactant, anaqueous liquid carrier fluid, a rheology enhancer and a shear activationadditive. The fluid mixture has rheological properties wherein themixture exhibits shear-thickening behavior when the shear rate isincreased from a first shear rate to a second higher shear rate that isfrom about 10 s⁻¹ to about 1000 s⁻¹. In certain embodiments, the firstshear rate may be from about 10 s⁻¹ to about 100 s⁻¹. The fluid mixtureis introduced into a wellbore formed in a subterranean formation.

In certain embodiments, the viscoelastic surfactant is present in theliquid carrier fluid in a state wherein the viscoelastic surfactantexhibits general phase separation within the carrier fluid when atstatic surface conditions. The fluid mixture may be introduced into awellbore as at least one of wellbore cleanup fluid, a diverter fluid, afracturing fluid and a fluid loss pill.

The fluid mixture may further comprise a rheology enhancer. The rheologyenhancer may be selected from at least one of a polyvinyl ester, apolyvinyl acetate, a polyacrylate, a polyvinyl alcohol, an aromaticsulfonate, mutual solvents, ABA copolymers, and combinations thereof.

The viscoelastic surfactant may be selected from a zwitterionicsurfactant, an amphoteric surfactant, an anionic surfactant, a cationicsurfactant, and combinations thereof. The zwitterionic surfactant maycomprise a surfactant or mixture of surfactants having the formula:

RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH₂)_(b)—N⁺(CH₃)₂—(CH₂)_(a)′(CH₂CH₂O)_(m)′(CH₂)_(b)′COO⁻

wherein R is an alkyl group that contains from about 17 to about 23carbon atoms which may be branched or straight chained and which may besaturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and mand m′ are each from 0 to 13, a and b are each 1 or 2 if m is not 0 and(a+b) is from 2 to 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is 0and (a′+b′) is from 1 to 5 if m′ is 0; (m+m′) is from 0 to 14; andCH₂CH₂O may also be OCH₂CH₂.

The fluid mixture may further comprise a shear activation additive thatinteracts with the viscoelastic surfactant to facilitate theshear-thickening behavior. In certain applications, the shear activationadditive may be a delayed shear activation additive so that shearactivation additive interacts with the viscoelastic surfactantsubsequent to the fluid mixture being introduced into the wellbore.Where the viscoelastic surfactant is a zwitterionic surfactant, theshear activation additive may be a base.

In certain applications, the fluid mixture introduced into the wellboreis brought to the surface and reintroduced into the same or a differentwellbore. In some cases, the fluid mixture introduced into the wellboreis allowed to entrain particles from the well within the fluid mixture.The fluid mixture is brought to the surface at a flow rate thatfacilitates the shear thickening behavior when brought to the surfacewhile the particles are entrained therein. The fluid mixture is allowedto rest after being brought to the surface so that the viscosity of thefluid mixture falls below the viscosity exhibited during the shearthickening behavior so that the entrained particles settle within thefluid mixture. The fluid mixture may then be reintroduced into the sameor a different wellbore after separating the settled particles from thefluid mixture.

A well treatment fluid is also provided in accordance with theembodiments. The well treatment fluid is a fluid mixture of aviscoelastic surfactant and a liquid carrier fluid. The fluid mixturehas rheological properties wherein the mixture exhibits shear-thickeningbehavior when the shear rate is increased from a first shear rate to asecond higher shear rate. In some embodiments, the first shear rate isfrom about 10 s⁻¹ to about 100 s⁻¹.

In certain embodiments, the viscoelastic surfactant is present in theliquid carrier fluid in a state wherein the viscoelastic surfactantexhibits general phase separation within the carrier fluid when atstatic surface conditions.

The fluid mixture may further comprise a rheology enhancer. The rheologyenhancer may be selected from at least one of a polyvinyl ester, apolyvinyl acetate, a polyacrylate, a polyvinyl alcohol, an aromaticsulfonate, mutual solvents, ABA copolymers, and combinations thereof.

The viscoelastic surfactant may be selected from a zwitterionicsurfactant, an amphoteric surfactant, an anionic surfactant, a cationicsurfactant, and combinations thereof. The zwitterionic surfactant maycomprise a surfactant or mixture of surfactants having the formula:)

RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH²)_(b)—N⁺(CH₃)₂—(CH₂)_(a)′(CH₂CH₂O)_(m)′(CH₂)_(b)′COO⁻

wherein R is an alkyl group that contains from about 17 to about 23carbon atoms which may be branched or straight chained and which may besaturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and mand m′ are each from 0 to 13, a and b are each 1 or 2 if m is not 0 and(a+b) is from 2 to 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is 0and (a′+b′) is from 1 to 5 if m′ is 0; (m+m′) is from 0 to 14; andCH₂CH₂O may also be OCH₂CH₂.

The fluid mixture may further comprise a shear activation additive thatinteracts with the viscoelastic surfactant to facilitate theshear-thickening behavior. In certain embodiments, the shear activationadditive may be a delayed shear activation additive so that shearactivation additive interacts with the viscoelastic surfactantsubsequent to the fluid mixture being introduced into the wellbore.Where the viscoelastic surfactant is a zwitterionic surfactant, theshear activation additive may be a base.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, reference is nowmade to the following descriptions taken in conjunction with theaccompanying figures, in which:

FIG. 1 is a representation of viscoelastic surfactant molecules arrangedin a spherical micelle arrangement and a lamella arrangement;

FIG. 2 is a shear sweep plot of viscosity versus shear rate for fluidSample 1 from Example 1;

FIG. 3 is a shear sweep plot of viscosity versus shear rate for fluidSample 2 from Example 1;

FIG. 4 is a plot of viscosity over time for fluid Sample 1 from Example2 in a shear-thickened state at shear rates of 10 s⁻¹ and 1000 s⁻¹; and

FIG. 5 is a plot of viscosity over time of fluid Sample 1 sheared at1000 s⁻¹ from Example 3, wherein the fluid is allowed to relax at ashear rate of 1 s⁻¹.

DETAILED DESCRIPTION

The description and examples are presented solely for the purpose ofillustrating the different embodiments and should not be construed as alimitation to the scope and applicability of the invention. While anycompositions may be described herein as comprising certain materials, itshould be understood that the composition could optionally comprise twoor more chemically different materials. In addition, the compositionscan also comprise some components other than the ones already cited.While the embodiments may be described in terms of treatment of verticalwells, it is equally applicable to wells of any orientation. Theembodiments may be described for hydrocarbon production wells, but it isto be understood that the invention may be used for wells for productionof other fluids, such as water or carbon dioxide, or, for example, forinjection or storage wells. It should also be understood that throughoutthis specification, when a concentration or amount range is described asbeing useful, or suitable, or the like, it is intended that any andevery concentration or amount within the range, including the endpoints, is to be considered as having been stated. Furthermore, eachnumerical value should be read once as modified by the term “about”(unless already expressly so modified) and then read again as not to beso modified unless otherwise stated in context. For example, “a range offrom 1 to 10” is to be read as indicating each and every possible numberalong the continuum between about 1 and about 10. In other words, when acertain range is expressed, even if only a few specific data points areexplicitly identified or referred to within the range, or even when nodata points are referred to within the range, it is to be understoodthat the inventors appreciate and understand that any and all datapoints within the range are to be considered to have been specified, andthat the inventors have possession of the entire range and all pointswithin the range.

There may be certain applications in the treatment of wells where it maybe desired to pump a thin fluid and then rely on a trigger to make thefluid into a viscous or viscoelastic fluid. The present embodiments aredirected towards such fluids and methods of using such fluids.

Instead of behaving like traditional VES fluids, where the fluid is lessviscous at higher shear, the present embodiments provide a fluid thathas higher viscosity when shear is increased. These fluids deviate fromthe log-linear power-law behavior of traditional VES fluids by givingthe same viscosity or a higher viscosity as the shear rate is increased.This may be accomplished by providing a VES fluid wherein theviscoelastic surfactant is present in a liquid carrier fluid in a statewherein the hydrophilic-lipophilic balance (HLB) of the viscoelasticsurfactant is such that the VES is on the verge of forming worm-likemicelles when the fluid is at static conditions. In certain instances,the VES may be in such a state that it exhibits phase separation fromthe carrier fluid when at static surface conditions. As used herein, theexpression “static conditions” is meant to encompass a fluid that hasbeen at rest or has not been subjected to shear for more than 30minutes. As used herein, the expression “static surface conditions” ismeant to encompass a fluid that has been at rest or not subjected toshear for more than 30 minutes at those conditions existing at thesurface of the well with which the fluid is used. Such surfaceconditions may include an atmospheric pressure of approximately oneatmosphere and a temperature of from about 5° C.-30° C., although thesurface conditions for each particular well may vary for each well.

The viscoelastic surfactant may be a zwitterionic surfactant, a cationicsurfactant, an amphoteric surfactant, an anionic surfactant, orcombinations of these. Zwitterionic surfactants may be particularlyuseful. Suitable zwitterionic surfactants may have the structure ofFormula (1) below:

RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH₂)_(b)—N⁺(CH₃)₂—(CH₂)_(a)′(CH₂CH₂O)_(m)′(CH₂)_(b)′COO⁻  (1)

in which R is an alkyl group that contains from about 17 to about 23carbon atoms which may be branched or straight chained and which may besaturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and mand m′ are each from 0 to 13; a and b are each 1 or 2 if m is not 0 and(a+b) is from 2 to 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is 0and (a′+b′) is from 1 to 5 if m is 0; (m+m′) is from 0 to 14; andCH₂CH₂O may also be OCH₂CH₂.

Those zwitterionic surfactants that include betaines may be particularlyuseful. Two suitable examples of betaines are BET-O and BET-E. Thesurfactant in BET-O-30 is shown below; one chemical name isoleylamidopropyl betaine. It is designated BET-O-30 because as obtainedfrom the supplier (Rhodia, Inc., Cranbury, N.J., U.S.A.) it is calledMirataine BET-O-30 because it contains an oleyl acid amide group(including a C₁₇H₃₃ alkene tail group) and contains about 30% activesurfactant. The remainder is substantially water, sodium chloride, andpropylene glycol. The surfactant in BET-O-30 is shown in Formula (2)below:

An analogous material, BET-E-40, also available from Rhodia, Inc.,contains an erucic acid amide group (including a C₂₁H₄₁ alkene tailgroup) and is approximately 40% active ingredient, with the remainderbeing substantially water, sodium chloride, and isopropanol. Thesurfactant in BET-E-40 is shown in Formula (3) below:

One chemical name for this surfactant is erucylamidopropyl betaine.As-received concentrate of BET-E-40 was used in the experimentsdescribed later on. BET surfactants, and other VES's that may besuitable for the present embodiments, are described in U.S. Pat. No.6,258,859, which is incorporated herein by reference. Mixtures ofbetaines, especially BET-E-40 with other surfactants, may also besuitable.

Cationic viscoelastic surfactants may also be used. The cationicsurfactants may include the amine salts and quaternary amine saltsdisclosed in U.S. Pat. Nos. 5,979,557, and 6,435,277, which are eachincorporated herein by reference. Examples of suitable cationicviscoelastic surfactants include cationic surfactants that may have thestructure according to Formula (4) below:

R₁N⁺(R₂)(R₃)(R₄)X⁻  (4)

in which R₁ has from about 14 to about 26 carbon atoms and may bebranched or straight chained, aromatic, saturated or unsaturated, andmay contain a carbonyl, an amide, a retroamide, an imide, a urea, or anamine; R₂ , R₃, and R₄ are each independently hydrogen or a C₁ to aboutC₆ aliphatic group which may be the same or different, branched orstraight chained, saturated or unsaturated and one or more than one ofwhich may be substituted with a group that renders the R₂, R₃, and R₄group more hydrophilic; the R₂, R₃ and R₄ groups may be incorporatedinto a heterocyclic 5- or 6-member ring structure which includes thenitrogen atom; the R₂, R₃ and R₄ groups may be the same or different;R₁, R₂, R₃ and/or R₄ may contain one or more ethylene oxide and/orpropylene oxide units; and X⁻ is an anion. Mixtures of such compoundsare also suitable. As a further example, R₁ is from about 18 to about 22carbon atoms and may contain a carbonyl, an amide, or an amine, and R₂,R₃, and R₄ are the same as one another and contain from 1 to about 3carbon atoms.

Cationic surfactants having the structure R₁N⁺(R₂)(R₃)(R₄)X⁻ mayoptionally contain amines having the structure R₁N(R₂)(R₃). It is wellknown that commercially available cationic quaternary amine surfactantsoften contain the corresponding amines (in which R₁, R₂, and R₃ in thecationic surfactant and in the amine have the same structure). Asreceived commercially available VES surfactant concentrate formulations,for example cationic VES surfactant formulations, may also optionallycontain one or more members of the group consisting of alcohols,glycols, organic salts, chelating agents, solvents, mutual solvents,organic acids, organic acid salts, inorganic salts, oligomers, polymers,co-polymers, and mixtures of these members.

Another example of a cationic VES is crucyl bis(2-hydroxyethyl)methylammonium chloride, also known as (Z)-13 docosenyl-N—N-bis(2-hydroxyethyl)methyl ammonium chloride. It is commonly obtained frommanufacturers as a mixture containing about 60 weight percent surfactantin a mixture of isopropanol, ethylene glycol, and water. Other suitableamine salts and quaternary amine salts include (either alone or incombination in accordance with the invention), erucyl trimethyl ammoniumchloride; N-methyl-N,N-bis(2-hydroxyethyl) rapeseed ammonium chloride;oleyl methyl bis(hydroxyethyl)ammonium chloride;erucylamidopropyltrimethylamine chloride, octadecyl methylbis(hydroxyethyl) ammonium bromide; octadecyl tris(hydroxyethyl)ammoniumbromide; octadecyl dimethyl hydroxyethyl ammonium bromide; cetyldimethyl hydroxyethyl ammonium bromide; cetyl methylbis(hydroxyethyl)ammonium salicylate; cetyl methylbis(hydroxyethyl)ammonium 3,4,-dichlorobenzoate; cetyltris(hydroxyethyl)ammonium iodide; cosyl dimethyl hydroxyethyl ammoniumbromide; cosyl methyl bis(hydroxyethyl)ammonium chloride; cosyltris(hydroxyethyl)ammonium bromide; dicosyl dimethyl hydroxyethylammonium bromide; dicosyl methyl bis(hydroxyethyl)ammonium chloride;dicosyl tris(hydroxyethyl)ammonium bromide; hexadecyl ethylbis(hydroxyethyl)ammonium chloride; hexadecyl isopropylbis(hydroxyethyl)ammonium iodide; and cetylamino, N-octadecyl pyridiniumchloride.

Many fluids made with viscoelastic surfactant systems, for example thosecontaining cationic surfactants having structures similar to that oferucyl bis(2-hydroxyethyl)methyl ammonium chloride, inherently haveshort re-heal times and rheology enhancers, as described later on, maynot be needed except under special circumstances, for example at verylow temperature.

Amphoteric viscoelastic surfactants may also be used. Amhphotericviscoelastic surfactants can be modified to provide shear thickeningbehavior by making the VES less hydrophilic, such as keeping the pHhigh. Exemplary amphoteric viscoelastic surfactant systems include thosedescribed in U.S. Pat. No. 6,703,352, incorporated herein by reference,for example amine oxides. Other exemplary viscoelastic surfactantsystems include those described in U.S. Pat. Nos. 7,060,661 and7,303,018, each of which is incorporated herein by reference, forexample amidoamine oxides.

Anionic viscoelastic surfactants may also be used. The anionicviscoelastic surfactant may have the general formula R—X, where R is ahydrophobic chain and X is a carboxyl (—COO—), sulfonyl (—OSO₃—) orsulfuric (—SO₄—) group. In some embodiments, the anionic surfactant maybe an alkyl sarcosinate. The alkyl sarcosinate can generally have anynumber of carbon atoms. Certain alkyl sarcosinates have about 12 toabout 24 carbon atoms. In some embodiments the alkyl sarcosinate mayhave about 14 to about 18 carbon atoms. Specific examples of the numberof carbon atoms include 12, 14, 16, 18, 20, 22, and 24 carbon atoms. Thealkyl sarcosinate anionic surfactant may be represented by the chemicalFormula (5) below:

R₁CON(R₂)CH₂X   (5)

wherein R₁ is a hydrophobic chain having about 12 to about 24 carbonatoms, R₂ is hydrogen, methyl, ethyl, propyl, or butyl, and X iscarboxyl or sulfonyl. The hydrophobic chain can be an alkyl group, analkenyl group, an alkylarylalkyl group, or an alkoxyalkyl group.Specific examples of the hydrophobic chain include a tetradecyl group, ahexadecyl group, an octadecentyl group, an octadecyl group, and adocosenoic group.

The amount of VES incorporated into the fluids may vary depending uponthe use and the desired characteristics of the fluid. Generally, the VESmay be incorporated into fluids used in embodiments in an amount rangingfrom about 0.2% to about 15% by weight of total weight of fluid. Incertain embodiments, the VES may be present in an amount of from about0.5% to about 12% by weight of total weight of fluid. In still otherembodiments, the amount of VES may range from about 2% to about 10% byweight of total weight of fluid. The lower limit of VES should no lessthan about 0.2, 0.5, 0.7, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 14percent of total weight of fluid, and the upper limited being no morethan about 15 percent of total fluid weight, specifically no greaterthan about 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 1, 0.9, 0.7, 0.5 or0.3 percent of total weight of fluid.

Rheology enhancers may be used with the viscoelastic surfactants. Thesemay be used to shorten the shear recovery time, to increase theviscosity of the VES fluid and/or provide greater tolerance to brines.Such materials are typically called co-surfactants, rheology modifiers,rheology enhancers, etc. The expression “rheology enhancer,” however,shall be used herein to refer to such materials. Various rheologyenhancers may be used. The rheology enhancers are typically alcohols,organic acids, such as carboxylic acids and sulfonic acids, sulfonates,and others. In particular, such materials may include polyvinyl esters,polyvinyl acetates, polyacrylates, polyvinyl alcohols and aromaticsulfonates. Other non-limiting examples include those described in U.S.Pat. Nos. 7,402,549 and 7,507,693, each of which is incorporated byreference. U.S. Pat. No. 6,258,859 describes the use of sodiumdodecylbenzene sulfonate (SDBS) as a rheology enhancer. Still othersuitable rheology enhancers are certain chelating agents such astrisodium hydroxyethylethylenediamine triacetate.

Many amphiphilic materials may be used as rheology enhancers. These mayinclude mutual solvents and ABA copolymers with A and B having anopposite affinity to water or oil. Such materials are described in U.S.Pat. Nos. 7,320,952; 7,341,980; 7,387,986 and 7,402,549, which are eachincorporated herein by reference.

The amount and type of rheology enhancer may be dependent upon the VESsurfactant fluid system at a given concentration and temperature. Theappropriate amounts of surfactant and rheology enhancer are thosenecessary to achieve the desired stability and shear reheal time as maybe determined by experiment. The rheology enhancer may be used in thefluid system in similar amounts to those described in the patentsreferenced above.

In certain instances, it may be necessary to include a shear activationadditive. As used herein, the expression “shear activation additive” ismeant to encompass those compounds or additives that are added to thefluid wherein such compounds or additives interact with the viscoelasticsurfactant to facilitate the shear-thickening behavior, as is describedherein. These may include acids, bases, salts, solvents, mutualsolvents, etc. The type of shear activation additive used and amountsmay vary depending upon the type of viscoelastic surfactant used and itsstructure. In certain embodiments, a combination of differentviscoelastic surfactants that interact (e.g. cationic and anionic VES)to provide a similar effect. The shear activation additive may includethose compounds that can facilitate the VES phase change from othernon-worm-like-micelle structures to worm-like micelles. In certainembodiments, environmental conditions may facilitate shear activationbehavior, such as temperature conditions.

To illustrate an example of how such shear activation additive may beused, it has been found that a zwitterionic surfactant with apermanently cationic quaternary amine in the middle part of thehydrophilic head and a carboxylic group on the front of the head can betuned to provide shear thickening rheological properties by increasingthe pH of the fluid. In such case, the shear activation additive is abase for increasing the pH of the fluid to provide the shear thickeningproperties. Not to be limited by theory, but it is believed that at verylow pH, the carboxylic group is completely protonated, leaving the wholemolecule acting like a normal cationic surfactant. At a pH between about6 to 9, most of the carboxylic groups are deprotonated so that theyexist as a carboxylate, wherein the molecule is slightly positivelycharged. At a very high pH (i.e. >10), however, the carboxylic groupsare all deprotonated and the charge is fully developed so that the wholemolecule forming the normally hydrophilic head is completely neutral. Inthe first two cases, the surfactants tend to arrange into worm-likemicelles to minimize the hydrophobic tail exposure to water. At veryhigh pH, however, when the fluid is first mixed, the surfactant canarrange into micelles, which may contain a certain amount of worm-likemicelles and some spherical micelles and even vesicles, with thepresence of worm-like micelles, the fluid acts as a viscoelastic fluid.When resting, however, the ionic interaction may be stronger to overcomethe hydrophobic interactions, so that the surfactants can arrange intospherical micelles to minimize charge repulsions between adjacent layersof surfactant, or to arrange in a semi-crystalline lamella phase thathas all the head groups paired with tails oriented in oppositedirections, as illustrated in FIG. 1. In this phase, because there areno long range 3-D interactions, such as occurs with worm-like orrod-like micelles, the fluid does not have viscoelasticity.

It can therefore be seen, when such fluids are at rest or there is lessshear applied to such fluids, the formation of either spherical micellesor the lamella structure, as in FIG. 1, may reduce the viscosity of thefluid. When a shear or a higher level of shear is applied, however, suchspherical micelles or lamella structures may be disrupted so that thesurfactant molecules may interact to form the worm-like micelles thatcontribute to viscoelasticity and impart higher viscosity to the fluid.Similar behaviors may be derived from other non-zwitterionicsurfactants. The shear activation additive may be selected and used inamounts that provide similar behaviors. One indication of such behavioris the solubility of the viscoelastic surfactant in the carrier fluid.The shear activation additives may modify the viscoelastic surfactantfluid so that the viscoelastic surfactant is substantially insolublewithin the carrier fluid or on the verge of insolubility as exhibited byphase separation of the viscoelastic surfactant from the aqueous carrierfluid when the fluid is at rest.

The shear activation additive may be a delayed shear activationadditive. The delayed shear activation additive interacts with theviscoelastic surfactant subsequent to the fluid mixture being introducedinto the wellbore. This facilitates maintaining the fluid at its normalviscoelastic state for longer periods, even when the fluid encountershigh shear conditions that are above the shear activation shearconditions. The delayed shear activation additive may include thosematerials previously discussed that may be encapsulated so that theirrelease is delayed or that may be derived in situ within the well afterbeing introduced.

The shear activation additive may be encapsulated within variousmaterials to facilitate delayed release. Some non-limiting examples ofencapsulating materials that may degrade or decompose to release theshear activation additive may include acrylics, halocarbon, polyvinylalcohol, Aquacoat® aqueous dispersions, hydrocarbon resins, polyvinylchloride, Aquateric® enteric coatings, hydroxypropyl cellulose (HPC),polyvinylacetate phthalate, hydroxypropyl methyl cellulose (HPMC),polyvinylidene chloride, hydroxylpropyl methyl cellulose phthalate(HPMCP), proteins, Kynar®, fluoroplastics, rubber (natural orsynthetic), caseinates, maltodextrins, shellac, chlorinated rubber,silicone, polyvinyl acetate phtalate (e.g. Coateric®) coatings,microcrystalline wax, starches, coating butters, milk solids, stearines,polyvinyl dichloride (Daran®) latex, molasses, sucrose, dextrins, nylon,surfactants, Opadry® combined polymer/plasticizer coating systems,Surelease® coating systems which are combination of film-formingpolymer; plasticizer and stabilizers for sustained release, enterics,paraffin wax, Teflon® fluorocarbons, Eudragits® polymethacrylates,phenolics, waxes, ethoxylated vinyl alcohol, vinyl alcohol copolymer,polylactides, zein, fats, polyamino acids, fatty acids, polyethylenegelatin, polyethylene glycol, glycerides, polyvinyl acetate, vegetablegums and polyvinyl pyrrolidone.

In other embodiments, the shear activation additive is formed in situ.These may include slowly dissolving solid materials, which may includeslowly dissolving acids, bases, salts, solvents, etc. One example ofsuch a material is MgO that slowly dissolves in water to form the baseMg(OH)₂ to facilitate raising the pH of the fluid. Urea is anotherexample of a material that will slowly dissolve in water to raise the pHof the fluid.

In situations where the shear activation additive is a pH loweringmaterial, the delayed shear activation additive may be a solid polymeracid precursor that dissolves in the fluids to form monomeric acids thatthen facilitate shear activation. The pH lowering materials may be used,for example, with anionic viscoelastic surfactants. Such solid polymeracid precursors may include homopolymers of lactic acid, glycolic acid,hydroxybutyrate, hydroxyvalerate and epsilon caprolactone, randomcopolymers of at least two of lactic acid, glycolic acid,hydroxybutyrate, hydroxyvalerate, epsilon caprolactone, L-serine,L-threonine, L-tyrosine, block copolymers of at least two ofpolyglycolic acid, polylactic acid, hydroxybutyrate, hydroxyvalerate,epsilon caprolactone, L-serine, L-threonine, L-tyrosine, homopolymers ofethylenetherephthalate (PET), butylenetherephthalate (PBT) andethylenenaphthalate (PEN), random copolymers of at least two ofethylenetherephthalate, butylenetherephthalate and ethylenenaphthalate,block copolymers of at least two of ethylenetherephthalate,butylenetherephthalate, ethylenenaphthalate and combinations of these.

In certain applications, the shear-activation additives that are insolid form may serve a dual function, serving not only to facilitateshear activation, but also to act like particulate materials that serveother functions, such as as a bridging agent when the fluid is acting asa diverter fluid. The shear-activation additive solid particles may beuse in combination with or as a mixture with other particulatematerials. The solid shear-activation additives may be a variety ofsizes, for example from about 8 mesh (2.38 mm) to about 170 mesh (0.088mm). The solid shear activation additives may also be formed in avariety of shapes, such as rods, spheres, flakes, fibers, etc.

In use, the VES treatment fluid may be formulated at the surface. Theviscoelastic surfactant, any rheology enhancer and/or shear activationadditive, and any other additives, may be mixed with an aqueous fluid,such as fresh water, sea water, brine (e.g. KCl, NaCl, Ca, etc.), etc.Such materials may be mixed in a batch operation or on the fly. Thetreatment fluid is then introduced into the wellbore of the formation tofacilitate treatment.

The shear-thickening VES fluid may exhibit a shear-thickening behaviorwhen the shear rate is increased from one level to another. At or aroundthe point at which the shear-thickening behavior is observed, the fluidmay be activated or triggered to provide a higher viscosity, even whenthe shear is backed down to a lower shear rate. The amount of shearrequired to provide this activation or trigger and the extent of shearthickening behavior may vary. The shear-thickening characteristics maybe designed for the particular treatment for which the treatment fluidis to be used and the shear typically encountered when performing theparticular treatment. In certain applications, the fluids may exhibitshear-thickening behavior when the shear rate is at about 10 s⁻¹ ormore. Thus, the fluids may undergo shear-thickening behavior when theshear rate is increased from a first shear rate in the range of about 1s⁻¹ to about 1000 s⁻¹ or more to another higher shear rate. In certainapplications, the shear-thickening behavior may be exhibited when theshear is initially from about 10 s⁻¹ or less to about 100 s⁻¹ so that athigher shear rates (i.e. greater than 100 s⁻¹) the shear-thickeningbehavior is observed. In other applications, the shear-thickeningbehavior may be observed when the shear is increased from about 1 s⁻¹ orless, with shear-thickening behavior occurring when the shear rate isincreased to 10 s⁻¹ or more. In some cases, the shear-thickeningbehavior may be triggered more rapidly at higher shear rates, whilelower shear rates may still trigger the shear-thickening behavior butrequire longer shear times.

The degree of shear thickening behavior for each fluid may vary. Incertain applications, once the fluid is subjected to the shear rate thattriggers shear-thickening behavior, the viscosity of the fluid may beincreased by as much as about 10%, 100%, 1000% or even 2000%. Theelevated viscosity may be maintained until the fluid is allowed to rest,where the fluid viscosity returns to the pre-shear-thickening viscosityfor any given shear rate. Resting of the fluid may be defined assubjecting the fluid to shear of about 1 s⁻¹ or less. This return orrecycling of the fluid to its initial state may occur without anychemical modification of the fluid wherein further additives are addedto the fluid to return the fluid to its pre-shear-thickened state.

The fluids have application to various well treatments. Non-limitingexamples may include use as a wellbore cleanup fluid, a diverter fluid,a fracturing fluid or a fluid loss pill. The fluids may have applicationto other treatments as well.

The fluids have particular application for use as wellbore cleanupfluids. As used herein, the expression “wellbore” is meant to encompassopen or cased holes or production tubing at any time between drillingand abandonment. Such wellbores may be vertical, deviated, orhorizontal. The wellbore cleanout may be carried out to remove variousparticles or materials. Particles, such as beads, may be used in jettingoperations in wellbores and afterward it may be desirable to remove thismaterial. Produced “sand” (including proppant and/or gravel and/orparticles such as silt and clay from the formation) accumulating in theproduction tubing during fluid flow to the wellbore can greatly restricthydrocarbon production. Other undesired particulates such as cuttings,cement pieces, fibers, other proppant flowback control agents, fluidloss additives, and debris, including debris formed by perforating, maybe present at some stage in the life of a well. In horizontal ordeviated wells, undesired fluids, that may or may not contain solidparticles, may accumulate in low spots.

Coiled tubing has been widely used as a means to clean out materialsfrom a wellbore. It is typically desirable for fluids used for coiledtubing cleanout to have low friction pressure to minimize hydraulichorsepower requirements and sufficient viscosity for good materialcarrying and/or displacing capability to aid in carrying and/ordisplacing the material to the surface for disposal. Normally, lightfluids are preferred to reduce the possibility of fracturing and tominimize the hydraulic horsepower needed to return the fluid to thesurface.

Normally in coiled tubing cleanout operations, the cleanout fluid ispumped down the coiled tubing, out through a specialized nozzle ornozzles at the end of the coiled tubing and into the wellbore, and thenup through the annulus between the coiled tubing and the wellbore. Thecleanout fluid fluidizes the particulate material and entrains it andcarries it to the surface. In a “reverse cleanout” operation, the fluidis circulated down through the annulus and up through the coiled tubingand no nozzles are used. In either case, the process may be carried outpressure-balanced to minimize fluid loss to the formation. In prior artmethods, when the fluid containing entrained particulates reaches thesurface, the particulates may be removed by a device such as asieve-shaker (shale shaker) or a centrifuge (hydrocyclone). Simplywaiting for the particles to separate out is not normally a suitablecommercial option because it takes too long in the viscous fluid neededto carry the particles to the surface. This is not necessary when usingthe shear-thickening fluids.

In use as a wellbore cleanup, when pumping the fluid downhole throughcoiled tubing, the fluid is initially in its pre-shear-thickened stateso that it is thin to reduce drag and minimize pumping horsepower. Whenthe fluid is jetted from the nozzle of the coil tubing, the shearexperienced during jetting through the nozzle activates or triggers theshear-thickening behavior so that it is viscous or viscoelastic tofacilitate transport of particles back through the wellbore up to thesurface. At the surface, when shear is removed from the fluid, itbecomes thin again to allow the particles to drop out of or be separatedfrom the fluid. The fluid can then be reused or recycled again into thesame or a different wellbore to facilitate further cleanup operations.

A suitable higher viscosity of the shear-thickened fluid is theviscosity necessary to fluidize the particles present downhole andentrain them and carry them to the surface. This depends among otherfactors upon the relative densities of the particles and the fluid, andon the flow rates at the nozzle and in the annulus. A suitable lowerviscosity is a viscosity at which the particles settle from the fluid atthe surface in a time short enough so that additional mechanical devicessuch as sieve shakers and centrifuges are not needed. This depends amongother factors upon the relative densities of the particles and thefluid, the residence time of the fluid in the settling tank, which maybe dependent on the flow rate and tank capacity, and the time and theremoval effectiveness the operator considers satisfactory. In general aviscosity of at least about 50 mPa·s, for example at least about 100mPa·s, is suitable for fluidizing and entraining particles in a cleanoutoperation. As presented herein, all viscosities are given at a shearrate of 100 sec⁻¹. An upper limit might be based on minimizingunnecessary expenditure of hydraulic horsepower in pumping. In general aviscosity of less than about 50 mPa·s, for example less than about 20mPa·s, may be suitable for particle settling and for initial pumpinginto the well.

In wellbore cleanouts, solid additives are normally not included in thecleanout fluids because they are removed when the particles are takenout of the fluid at the surface. If necessary, however, fluid lossadditives may be included as may fibers that improve the ability of afluid to entrain other particles. These may be selected to be of suchlow density that they are not removed in the settling stage, or they maybe removed and then fresh portions added in subsequent stages. Fibersmay not pass through the coiled tubing nozzles but may be used inreverse cleanouts. The components of the cleanout fluid may bebatch-mixed or mixed on the fly. If batch mixing is used, anycombination of components may be premixed in a concentrate or slurry fordelivery to the job site.

The fluids may also be used as diverter fluids. In hydraulic fracturing,for instance, when the pressure reaches a certain threshold,micro-fractures in the formation may open creating flow paths wherethere is a large amount of fluid loss. These lost fluids can damage theformation. The sudden high fluid loss can also cause the proppant tobridge during fracturing treatment, which can lead to screen-out. As thefluid leaks off into these fractures, it is suddenly high sheared. Ascan be seen, if the shear-thickening shear trigger of the fluids isreached due to this high shear, the higher viscosity or viscoelasticitymay facilitate temporarily sealing or reducing fluid leak off to thesefractures. The main fracture can then continue to propagate in its maindirection.

Once the fracture treatment is complete, or the shearing is removed, thefluid relaxes to its pre-triggered viscosity to facilitate flowback. Theviscoelastic surfactant fluid provides a solid-free environment so thatit ensures the formation is not damaged. In certain applications, theuse of the VES fluids as a diverter can facilitate diverting a treatmentfrom one set of perforations or zones to another when needed. Varioussolid materials, which may be degradable or non-degradable, such asproppant, fibers, dissolvable fluid loss additives, etc., may also beused in the treatment fluids.

For matrix acidizing treatments, fluids are injected into thesubterranean formation at pressures below fracturing pressures. It iscommon in many cases to use coiled tubing to carry out the stimulationtreatment. In such cases the shear-thickening fluid can be pumped downthe coiled tubing at any given time where there is a need to divertfluids away from zones that are already stimulated. A common jobsequence for such a stimulation treatment consists of alternating astimulation fluid with a diverter, stimulation fluid. These are normallyacid-based fluids (e.g. hydrochloric, formic, acetic etc) or can benon-acid based fluid like chelants. The objective of having reactingfluids treating the critical matrix of the reservoir is to bypass oreliminate the formation damage induced during the drilling andcompletion of the well. Diverters are commonly used for thoseapplications to ensure the stimulation fluid is distributed ashomogeneous as possible across the different sections of the reservoir.The shear-thickening fluids used in such cases are pumped downhole as adiverter, whereby the shear-activated fluid will experience an increaseof viscosity after passing through a downhole coiled tubing nozzle. Oncethe fluid is shear activated and distributed across the zones beingtreated to facilitate diversion, the fluid with increased viscosity willexhibit more resistance to injection and will force the subsequent fluidintroduced into an un-stimulated zone. One desirable characteristic ofparticulate or chemical diverters is that they leave no residue ordamage behind. In the present case, by using VES fluids no damage isleft behind.

Also when using the shear-activated fluid as a diverter, it is possibleto use other degradable particulates to enhance the bridging capabilityof the diverter fluid, especially when treating formations having alarge contrast of permeability or that are naturally fractured. Suchparticulates may be of different sizes and shapes, such as spheres,flakes, rods, fibers, etc. A mixture of different degradableparticulates could also be used depending on the permeability contrastof the formation. The shear-activation additives may also be used as theparticulate or in combination with other particulates, as previouslydiscussed.

The shear-thickening fluids may also be used for acid fracturingapplications. In such applications, it is common to alternate reactivefluids (acids, chelants etc) with non-reactive fluids (VES-fluids,polymer-based fluids) during the acid fracturing operations. In suchtreatments, the fluids are injected at pressure above fracturingpressures in carbonate (e.g. limestone and dolomite) formations. Onedesirable characteristic of the fluids used in acid fracturing is tohave a large viscosity contrast. The contrasts in viscosity of theinjected fluids causes the fluids to generate viscous fingering, wherebythe reactive fluid (normally having low viscosity) will preferentiallychannel or finger thru the fluid with higher viscosity. Suchpreferential flow extends the fingering inside the hydraulic fractureand may facilitate differential etching. Such differential etching isdirectly proportional to the conductivity (fracture permeability timefracture's width) generated by the acid fracturing. Shear activatedfluids may promote the fingering effects by providing a more viscousfluids after they are exposed to the shear at the perforations. Likewisein acid fracturing, the fluids may be polymer-and solids free.

In another application, the shear-thickening VES fluid is used in amanner similar to “slick-water” fracturing, where low viscosity fluids,typically water, are commonly used. In such fracturing operations, thelow viscosity fluid, which may be combined with a friction reducingagent, is pumped with low viscosity and low friction pressure throughthe tubing. Typically, polyacrylamides are used as the friction-reducingagent. Once the fluid passes the perforations, shear activates the fluidto increase the viscosity or cause it to become viscoelastic. Thethicker fluid facilitates carrying the proppant deeper into theformation.

In such treatments, lighter weight and significantly lower amounts ofproppant than conventional viscosified fracturing fluids may be used.The proppant used may have a smaller particle size than those used fromconventional fracturing treatments used in oil-bearing formations. Whereit is used, the proppant may have a size, amount and density so that itis efficiently carried, dispersed and positioned by the treatment fluidwithin the formed fractures. Proppants may include sand, bauxite,ceramic and other materials. Such fracturing fluids may be particularlyuseful in low-permeable, gas-bearing formations, such as tight-gas shaleand sand formations, where fracture width is of less concern. Suchformations may have a permeability of from about 1 mD or 0.5 mD or less.The treatment fluid is introduced into the formation above the fracturepressure of the formation. The friction-reducing agent may be apolyacrylamide, which are typically used in slick-water fracturingfluids.

The shear-thickening fluids may also be used as a fluid loss pill foruse in high leak off zones of the formation. They may be used where highdensity brines are being lost to the formation. In conventional fluidloss pills, a crosslinked polymer fluid may be used, which may bedifficult to clean up and may damage the formation. In the presentinvention, the fluid is a shear-thickening fluid that is in itsinitially low viscosity state prior to shear activation. The thicknessof the fluid is increased by the increased shear due to the leak offinto the formation. When the fluid relaxes, the viscosity lowers, whichallows the fluid to flow back from the high permeability zone. Theshear-thickening VES fluids used as a fluid loss pill may furthercomprise bridging particulates, usually graded sodium or potassiumsalts, sized calcium carbonate particulates, or even degradableparticulates like graded polylactide acid particles or fibers.

The shear-thickening fluids described herein may be used for otherapplications, as well. Although the uses have been described in terms ofproducing wells for oil and/or gas, the fluids and methods may also beused for injection wells (such as for enhanced recovery or for storage)or for production wells for other fluids such as carbon dioxide orwater.

The following examples serve to further illustrate the embodiments.

EXAMPLES Example 1

Sample 1: To form Sample 1, an aqueous VES fluid was formulated usingfresh water and the zwitterionic viscoelastic surfactant BET-E-40,available from Rhodia, Inc., which was approximately 40% as active VES,with the remainder being substantially water, sodium chloride, andisopropanol. The BET-E-40 was used in an amount of about 1.5% by weightof the fluid. A polyvinyl alcohol rheology enhancer (Alcotex WD100,available from Synthomer Inc., at 43% hydrolyzed PVA), was used in anamount of about 0.004% by weight of the fluid. Sodium hydroxide was usedas the shear activation additive at approximately 8% by weight of thefluid to provide a fluid pH of about 12 to 13.

Sample 2: Sample 2 was prepared similarly to Sample 1, but contained4.5% by weight BET-E-40 and the WD100 rheology enhancer was used in anamount of about 0.012% by weight of the fluid. Sodium hydroxide was usedas the shear activation additive at approximately 8% by weight of thefluid to provide a fluid pH of about 12 to 13.

Each of the fluid Samples 1 and 2 were blended in a Waring blender andallowed to sit for no less than 20 minutes. The fluids were thin with noelasticity and low viscosity. The mixtures were each loaded into aBohlin rheometer with a cup and vein tool geometry. The vein tool wasused to mimic mixing that was not simply laminar shear. The rheometerwas started in the viscometry mode from low shear rate to high, and thendown to low shear rate again to provide a shear sweep curve. The resultsare presented in FIGS. 2 and 3.

As can be seen from FIGS. 2 and 3, when the shear rate was around 10s⁻¹, the fluids deviated from the power law shear thinning behaviortypically observed for traditional VES fluids and started to increase inviscosity. This was an indication that shear thickening was occurringand the fluid was shear activated in its basic condition. The shear rateramping down curves of each fluid shows consistently higher viscositythan their viscosity during the shear rate ramping up at equivalentshear rates. This also indicates that the high shear in the shear rateramp up had already activated the fluid to a higher viscosity orimproved viscoelasticity than during the fluid shear ramp up.

Visual observations of each of fluid Samples 1 and 2 confirmedqualitatively the properties of the shear-thickening fluids. When eitherof the fluids was sheared at high shear rates for a short time (lessthan 1 minute) or at lower shear rates for longer periods (no less than5 minutes), they became viscoelastic, as evidenced by the formation of alip when the fluids were poured from one container to another. Uponsitting statically in the blender cup, the fluids gradually lost theirlipping ability and eventually became thin, pouring like water. ForSample 2, which contained 4.5% of the VES, after sitting for greaterthan two hours, there was phase separation observed with a topsurfactant rich layer resting on a thin bottom water layer. Sample 1 didnot exhibit phase separation even after resting overnight.

Example 2

Two fully relaxed loadings of Sample 1 from Example 1 were each loadedin the Bohlin rheometer with the geometry previously described forExample 1. The rheometer was started at room temperature in viscometrymode at shear rates of 10 s⁻¹ and 1000 s⁻¹. The shear rates were appliedfor four minutes for each loading. Viscosities were recorded as thefluid was being sheared. The results are presented in FIG. 4.

As can be seen from FIG. 4, the fluid viscosities increased at bothshear rates, indicating that shear thickening or gelling had occurred.The fluid viscosity at the higher shear rate at 1000 s⁻¹ was higher thanthe viscosity at 10 s⁻¹, indicating that the fluid is a shear thickeningfluid. Also, when the fluid was sheared at 1000 s⁻¹, it was activatedmore rapidly than when a shear of 10 s⁻¹ was applied. This was likelythe result of the shear energy input to the fluid.

Example 3

After the fluid sample sheared at 1000 s⁻¹ from Example 2 was shearedfor a few minutes, it was allowed to relax at a low shear rate of 1 s⁻¹.This was done to record the viscosity change profile. The results arepresented in FIG. 5. The viscosity started at a high value of about 0.24Pa·s after high shear. The viscosity then gradually dropped down to0.026 Pa·s, which is an order of magnitude decrease. This is evidencethat the shear-activated fluid was breaking with time.

While the invention has been shown in only some of its forms, it shouldbe apparent to those skilled in the art that it is not so limited, butis susceptible to various changes and modifications without departingfrom the scope of the invention. Accordingly, it is appropriate that theappended claims be construed broadly and in a manner consistent with thescope of the invention.

1. A method comprising: providing a fluid mixture of a viscoelastic surfactant and a liquid carrier fluid, the fluid mixture having rheological properties wherein the mixture exhibits shear-thickening behavior when the shear rate is increased from a first shear rate to a second higher shear rate; and introducing the fluid mixture into a wellbore formed in a subterranean formation.
 2. The method of claim 1, wherein: the viscoelastic surfactant is present in the liquid carrier fluid in a state wherein the viscoelastic surfactant exhibits general phase separation within the carrier fluid when at static surface conditions.
 3. The method of claim 1, wherein: the fluid mixture is introduced into a wellbore as at least one of wellbore cleanup fluid, a diverter fluid, a fracturing fluid and a fluid loss pill.
 4. The method of claim 1, wherein: the first shear rate is from about 10 s⁻¹ to about 100 s⁻¹.
 5. The method of claim 1, wherein: the fluid mixture further comprises a rheology enhancer.
 6. The method of claim 5, wherein: the rheology enhancer is selected from at least one of a polyvinyl ester, a polyvinyl acetate, a polyacrylate, a polyvinyl alcohol, an aromatic sulfonate, mutual solvents, ABA copolymers, and combinations thereof.
 7. The method of claim 1, wherein: the viscoelastic surfactant is selected from a zwitterionic surfactant, an amphoteric surfactant, an anionic surfactant, a cationic surfactant, and combinations thereof.
 8. The method of claim 1, wherein: the viscoelastic surfactant is a zwitterionic surfactant.
 9. The method of claim 8, wherein: the zwitterionic surfactant comprises a surfactant or mixture of surfactants having the formula: RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH₂)_(b)—N⁺(CH₃)₂—(CH₂)_(a)′(CH₂CH₂O)_(m)′(CH₂)_(b)′COO⁻ wherein R is an alkyl group that contains from about 17 to about 23 carbon atoms which may be branched or straight chained and which may be saturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and m and m′ are each from 0 to 13, a and b are each 1 or 2 if m is not 0 and (a+b) is from 2 to 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is not 0 and (a′+b′) is from 1 to 5 if m′ is 0; (m+m′) is from 0 to 14; and CH₂CH₂O may also be OCH₂CH₂.
 10. The method of claim 1, wherein: the fluid mixture further comprises a shear activation additive that interacts with the viscoelastic surfactant to facilitate the shear-thickening behavior.
 11. The method of claim 10, wherein: the shear activation additive is a delayed shear activation additive so that shear activation additive interacts with the viscoelastic surfactant subsequent to the fluid mixture being introduced into the wellbore.
 12. The method of claim 10, wherein: the viscoelastic surfactant is a zwitterionic surfactant and wherein the shear activation additive is a base.
 13. The method of claim 1, further comprising: bringing the fluid mixture introduced into the wellbore to the surface; and reintroducing the fluid mixture into the same or a different wellbore.
 14. The method of claim 1, wherein: the fluid mixture introduced into the wellbore is allowed to entrain particles from the well within the fluid mixture, the fluid mixture being brought to the surface at a flow rate that facilitates the shear thickening behavior when brought to the surface while the particles are entrained therein; and allowing the fluid mixture to rest after being brought to the surface so that the viscosity of the fluid mixture falls below the viscosity exhibited during the shear thickening behavior so that the entrained particles settle within the fluid mixture; and reintroducing the fluid mixture into the same or a different wellbore after separating the settled particles from the fluid mixture.
 15. A well treatment method comprising: providing a fluid mixture of a zwitterionic viscoelastic surfactant, an aqueous liquid carrier fluid, a rheology enhancer and a shear activation additive, the fluid mixture having rheological properties wherein the mixture exhibits shear-thickening behavior when the shear rate is increased from a first shear rate to a second higher shear rate that is from about 10 s⁻¹ to about 1000 s⁻¹; and introducing the fluid mixture into a wellbore formed in a subterranean formation.
 16. The method of claim 15, wherein: the viscoelastic surfactant is present in the liquid carrier fluid in a state wherein the viscoelastic surfactant exhibits general phase separation within the carrier fluid when at static surface conditions.
 17. The method of claim 15, wherein: the first shear rate is from about 10 s⁻¹ to about 100 s⁻¹.
 18. The method of claim 15, wherein: the fluid mixture further comprises a rheology enhancer.
 19. The method of claim 18, wherein: the rheology enhancer is selected from at least one of a polyvinyl ester, a polyvinyl acetate, a polyacrylate, a polyvinyl alcohol, an aromatic sulfonate, mutual solvents, ABA copolymers, and combinations thereof.
 20. The method of claim 15, wherein: the viscoelastic surfactant is selected from a zwitterionic surfactant, an amphoteric surfactant, an anionic surfactant, a cationic surfactant, and combinations thereof.
 21. The method of claim 15, wherein: the viscoelastic surfactant is a zwitterionic surfactant.
 22. The method of claim 21, wherein: the zwitterionic surfactant comprises a surfactant or mixture of surfactants having the formula:) RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH²)_(b)—N⁺(CH₃)₂—(CH₂)_(a)′(CH₂CH₂O)_(m)′(CH₂)_(b)′COO⁻ wherein R is an alkyl group that contains from about 17 to about 23 carbon atoms which may be branched or straight chained and which may be saturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and m and m′ are each from 0 to 13, a and b are each 1 or 2 if m is not 0 and (a+b) is from 2 to 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is not 0 and (a′+b′) is from 1 to 5 if m′ is 0; (m+m′) is from 0 to 14; and CH₂CH₂O may also be OCH₂CH₂.
 23. The method of claim 15, wherein: the mixture further comprises a shear activation additive that interacts with the viscoelastic surfactant to facilitate the shear-thickening behavior.
 24. The method of claim 23, wherein: the shear activation additive is a delayed shear activation additive so that shear activation additive interacts with the viscoelastic surfactant subsequent to the fluid mixture being introduced into the wellbore.
 25. The method of claim 15, wherein: the viscoelastic surfactant is a zwitterionic surfactant and wherein the shear activation additive is a base.
 26. The method of claim 15, wherein: the fluid mixture is introduced into a wellbore as at least one of wellbore cleanup fluid, a diverter fluid, a fracturing fluid and a fluid loss pill.
 27. The method of claim 15, further comprising: bringing the fluid mixture introduced into the wellbore to the surface; and reintroducing the fluid mixture into the same or a different wellbore.
 28. The method of claim 15, wherein: the fluid mixture introduced into the wellbore is allowed to entrain particles from the well within the fluid mixture, the fluid mixture being brought to the surface at a flow rate that facilitates the shear thickening behavior when brought to the surface while the particles are entrained therein; and allowing the fluid mixture to rest after being brought to the surface so that the viscosity of the fluid mixture falls below the viscosity exhibited during the shear thickening behavior so that the entrained particles settle within the fluid mixture; and reintroducing the fluid mixture into the same or a different wellbore after separating the settled particles from the fluid mixture.
 29. A well treatment fluid comprising: a fluid mixture of a viscoelastic surfactant and a liquid carrier fluid, the fluid mixture having rheological properties wherein the mixture exhibits shear-thickening behavior when the shear rate is increased from a first shear rate to a second higher shear rate.
 30. The well treatment fluid of claim 29, wherein: the viscoelastic surfactant is present in the liquid carrier fluid in a state wherein the viscoelastic surfactant exhibits general phase separation within the carrier fluid.
 31. The well treatment fluid of claim 29, wherein: the first shear rate is from about 10 s⁻¹ to about 100 s⁻¹.
 32. The well treatment fluid of claim 29, further comprising: a rheology enhancer.
 33. The well treatment fluid of claim 32, wherein: the rheology enhancer is selected from at least one of a polyvinyl ester, a polyvinyl acetate, a polyacrylate, a polyvinyl alcohol, an aromatic sulfonate, mutual solvents, ABA copolymers, and combinations thereof.
 34. The well treatment fluid of claim 29, wherein: the viscoelastic surfactant is selected from a zwitterionic surfactant, an amphoteric surfactant, an anionic surfactant, a cationic surfactant, and combinations thereof.
 35. The well treatment fluid of claim 29, wherein: the viscoelastic surfactant is a zwitterionic surfactant.
 36. The well treatment fluid of claim 35, wherein: the zwitterionic surfactant comprises a surfactant or mixture of surfactants having the formula:) RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH²)_(b)—N⁺(CH₃)₂—(CH₂)_(a)′(CH₂CH₂O)_(m)′(CH₂)_(b)′COO⁻ wherein R is an alkyl group that contains from about 17 to about 23 carbon atoms which may be branched or straight chained and which may be saturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and m and m′ are each from 0 to 13, a and b are each 1 or 2 if m is not 0 and (a+b) is from 2 to 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is not 0 and (a′+b′) is from 1 to 5 if m′ is 0; (m+m′) is from 0 to 14; and CH₂CH₂O may also be OCH₂CH₂.
 37. The well treatment fluid of claim 29, further comprising: a shear activation additive that interacts with the viscoelastic surfactant to facilitate the shear-thickening behavior.
 38. The well treatment fluid of claim 37, wherein: the viscoelastic surfactant is a zwitterionic surfactant and wherein the shear activation additive is a base.
 39. The well treatment fluid of claim 37, wherein: the shear activation additive is a delayed shear activation additive so that shear activation additive interacts with the viscoelastic surfactant subsequent to the fluid mixture being introduced into the wellbore. 